Monaco Bank is a submarine volcano located in the Azores Island Arc. The volcano was constructed on regional tectonic trend connecting the active volcanic island of Sao Miguel (5 volcanoes) to the now extinct Santa Maria Island. Like many Azores volcanoes, the volcano is rift-dominated, built on a NW-SE-trending fissure 20 km south of the western tip of Sao Miguel Island. The volcano therefore has an elongated profile (much like San Jorge Island, Azores) rising to within 197 metres of the sea surface.
Photo showing the sea floor around the Eastern Azores. Monaco Bank is the ridge extending south from Sao Miguel in the bottom centre of the image. Don Joao de Castro Bank rises from the sea floor between Sao Miguel and Terceira. Photo from Santos and Tempera et.al, 2010.
Two eruptions have been documented from Monaco Bank. The first, in 1907 was only discovered because it broke an underwater telegraph cable. In 1911, a second eruption occurred when underwater phreatic explosions created water jets the broke through the sea surface.
The volcano is unusual for a European volcano as it has never been studied. Unlike nearby Don Joao de Castro Bank seamount volcano, whose morphology and hydrothermal vent communities have been studied well.
R.S. Santos, F. Tempera, A. Colaço, F. Cardigos, and T. Morato. 2010. Mountains in the Sea, Spotlight 11. Dom João de Castro Seamount. Oceanography, 23:200–201, ( http://dx.doi.org/10.5670/oceanog.2010.83)
NOAA. 1986. Catalog of Submarine Volcanoes and Hydrological Phenomena Associated With Volcanic Events, January 1, 1900 to December 31, 1959. 45 p
Siebert L, Simkin, T. 2002-. Volcanoes of the world: an illustrated catalog of Holocene volcanoes and their eruptions: Smithsonian Institution, Global Volcanism Program. Digital Information Series GVP-3, (http://www.volcano.si.edu/world)
From Global Tectonic 3rd Ed.
“The stability of the boundaries between plates is dependent upon their relative velocity vectors. If a boundary is unstable it will exist only instantaneously and will immediately devolve into a stable configuration… A more complex and potentially unstable situation arises when three plates come into contact at a triple junction. Quadruple junctions are always unstable, and immediately devolve into a pair of stable triple junctions.”
There are three essential types of junctions that make up plate boundaries at triple junctions. Trenches, Ridges, and Transform faults. (convergent, spreading, transform). Whether a junction is stable or not, depends on the movement rates of the various plates that make up the junction. Some orientations are completely stable (Ridge, Ridge, Ridge) others are completely unstable (Transform, Transform, Transform). Others have special conditions in which they are stable, or else the junction will migrate down one of the interfaces until it meets a stable condition.
A good example of that is the San Andreas. When the Farallon plate made its final plunge under the North American plate, two types of triple junction formed. (“A quadruple junction existed momentarily at about 28 Ma, but this devolved immediately into two triple junctions “) A ridge-transform-trench, and a transform-transform-ridge. Each one propagated North or South until it met up with the Mendocino Fracture zone (a transform) or the Murray Fracture zone (another transform). Once two of the three legs of the junction formed a strait line, the junction became stable. (Over time, the Murray FZ has effectively become “locked” and other dynamics further south in the Gulf of California have taken over) Another example of the dynamics of these forces is the Alpine Fault of New Zealand. Though not a full on triple junction system, two separate plates are colliding head on. North of the Alpine fault the Pacific plate is being sliced and part of it drops under the Australian Plate (Zealandia) and south of the Alpine Fault, the Australian plate (Zealandia) is being sliced in two. In each case, the Alpine fault is the intermediary and has become the transform fault between the two opposing trenches. Flower Structures dominate the Alpine fault as portions of it lock and fail over time.
The Azores Triple junction is the intersection of the MAR, and the Terceira Rift. As a transform-transform-transform system, it is one of the most stable configurations that you can get. The problem is that as the dynamics of the three major plates come into play, they can easily upset the balance and turn any part of that junction into an unstable configuration. Once that happens, the junction will migrate around trying to find equilibrium. Toss in a hotspot, it turns into a mess. On geologic timescales, it is quite easy for one massive plate to suddenly change direction. This is evidenced by the Emperor Seamount chain and the Hawaiian island chain. About 45 to 48 million years ago the Pacific Plate changed motion by about 70°. Any plate margin or junction that it was part of would have felt this sudden shift in the prevailing forces.
Now when you take a look at the dynamics at the Azores, it’s pretty easy to see the lasting effects of changes in plate movement.
Take note of where the Monaco Bank is at on this plot. At one time, the East Azores Fracture Zone was likely the dominant transform fault all the way over to the MAR as part if the Pico Fracture Zone.
When you take a look at the dynamics at the Azores, it’s pretty easy to see the lasting effects of changes in these dynamics. From Global Tectonics about Ridge, Ridge, Transform: “Unstable, evolves to FFR” (Transform, Transform, Ridge) Or, as seems to be the case for the Azores, snap off a piece of crust and make it a Ridge, Ridge Ridge junction.
Supporting Material by GEOLURKING.
“Global Tectonic 3rd ed” Keary (deceased), Klepis, and Vine. John Wiley & Sons Ltd. ISBN 978-1-4051-0777-8
Dynamics of Mantle Flow Around The Azores Triple Junction: Constraints from bathymetry and gravity data Sankar Thesis Paper (2009)
Name that volcano riddle by Suzie!
Out of chaos came darkness. Out of order comes enlightenment.